It seems like Google is now sending out notifications to those who have been spammy with their rich snippets.

One webmaster posted a notification he received in the Google Webmaster Help forums of a manual action he received for "Spammy structured markup."

Here is the text of that notification:

Spammy structured markup

Markup on some pages on this site appears to use techniques such as marking up content that is invisible to users, marking up irrelevant or misleading content, and/or other manipulative behavior that violates Google's Rich Snippet Quality guidelines.

This is the first time I have seen a webmaster report getting a manual action sent to them for spammy structured markup.